Fluorescent structure



1938- .1. c. BATCHELOR 2,125,599

Patented Aug. 2, 1938 UNITED STATES PATENT OFFICE FLUORESCENT STRUCTUREJohn C. Batchelor,'New York, N. Y. Application February 8, 1935, SerialNo. 5,521 '1 Claims. (Cl. 250-275) My invention relates to a cathode raytube suitable for use as an image reproducer in television systems andthe like, and, more particularly, to improvements in the light producingstrucl ture in such tubes.

It is customary in the visual communication arts to provide as an imagereproducing member, a cathode ray tube having an electron gun and afluorescent screen so disposed in an evacuated l container that, incooperation with appropriate associated circuit elements, an image of aremotely analyzed object may be reproduced on the fluorescent screenunder the bombardment of an electron beam which is caused to scan thefluo- 15 rescent screen rhythmically.

In such tubes the fluorescent screen usually comprises a layer of finelydivided fluorescent particles deposited upon an interior wall of thecontainer of the cathode ray tube, and so disposed 20 that its interiorsurface may be bombarded by an electron beam from. the electron gun, andthe light produced by the bombardment may be viewed through the wall ofthe container from a point exterior to the container. Because of the 25fact that the index of refraction of the particles of fluorescentmaterial is appreciably different from that of the evacuated space inwhich they are placed, and substantial voids exist between the'particlesof fluorescent material, appreciable 30 impedance is encountered bylight generated at the interior surface of the fluorescent screen as itpasses through the screen toward the exterior of the tube. In thisdiscussion I have referred to the effects of internal refraction andreflection 35 within the fluorescent screen as beingof impedance to thetransmission of light because of the fact that these effects do actuallyprevent certain of the light from travelling through the screen.Moreover, such light as does succeed in 40 penetrating the fluorescentscreen is to an appreciable extent diffused because of these effects,and consequently, the definition of a reproduced image is impaired.

In an effort to overcome these harmful effects,

45 it has been proposed in the art to provide a cathode ray tube havinga fluorescent screen prepared on a metal or other target and the targetso disposed and the bulb so designed that an image produced on such afluorescent screen might be 50 viewed from the side of the fluorescentscreen from which the electronic bombardment occurs. Invariably,however, tubes designed for this purpose are not of symmetrical shapeand are not geometrical figures of rotation, and therefore the 55manufacturing of such envelopes involves sufllciently great cost andeffort of production that it has in general been thought advisable toadhere to the older method of viewing an image on the side of thefluorescent screen opposite that on which it is produced, and, in orderto compensate for the poorer efllciency of such a tube, to providecorrespondingly higher energy of bombardment. With the foregoing inmind, it is an object of my invention to provide a cathode ray tubehaving an envelope of great geometric simplicity and yet having afluorescent screen whereon an image may be viewed from the side of thescreen upon which it is generated.

This and other objects will be apparent from the following descriptionof my invention.

In accordance with my invention, I have provided a cathode ray tubehaving a fluorescent screen comprising a reticulated metallic member ata position normally occupied by a conventional fluorescent screen, andhaving electrical insulating material deposited on the side of saidmember adjacent the electron gun, and having a fluorescent materialdeposited on the side of said member remote from said gun.

In order to describe my invention more fully, attention is directed tothe accompanying drawing of which Figure 1 represents a sectional viewof a preferred form of my invention and Figure 2 represents an enlargedschematic sectional view of a portion of the fluorescent screen andenvelope of my tube.

Referring to Figure 1, a bulb 5 is prepared by introducing a metallicfilm 8, which may be silver deposited from a solution of silver nitratein accordance with the known method, in a uniform film upon the innersurface of the bulb 5 and terminating in a uniform circle l3 in theenlarged portion of the bulb, and in a uniform circle I in the neck I5of the bulb 5.

In the neck I5 is an electron gun l1 carried on the stem l5 andcomprising an accelerating anode III, a control electrode I l and anelectron emitting cathode l8, whereby an electron beam may be ejectedtoward the fluorescent screen I. For the purpose of clearerrepresentation, the gun I! is not shown in section.

The fluorescent screen I comprises a woven wire screen or perforatedmetallic member of any suitable metal such as nickel, copper, platinumor the like, having deposited upon its front side or surface l9 adjacentthe gun IT a thin film of insulating material such as calcium fluoride,which is capable of relatively little secondary emission underelectronic bombardment, and having deposited on the rear side or surface20 adjacent thewindow 6 of the bulb 5 a film of any suitable fluorescentmaterial such as willemite. The screen I is mounted'substantiallyparallel and closeto the window E of the tube 5 and is carried by themetal ring 2 which is in turn supported by the lead-in wire3 in the stem4.

:In order to understand the construction and operation of my fluorescentscreen more clearly, it is convenient to refer to Figure 2 whichrepresents an enlarged schematic sectional view of the fluorescentscreen portion of my tube. this embodiment,a woven; wire screen is usedas a carrier for the: fluorescent material and the wires 2| representsuccessive wires in such a screen. On the side of the wires 2|approached by the eiectron beam represented by the arrow 2'2, isdeposited a film 23 of an insulating material. I have found itconvenient to app-By such a material by disposing the screen I in anevacuated container and evaporating. from a metal filament within theevacuated container, a sufficient quantity of insulating material, suchas calcium fluoride; at a point such that the molecules of: theinsulating material travel toward. and attach themselves to the surfaceof the secondary emission. In respect to all of these considerations,,Ihave found the alkaline halides to be satisfactory.

Following the deposition of the insulating material, a film 24 offluorescent material, such as finely divided :willemite, is depositedupon the side of the screen I opposite that carrying the insulatingmaterial film 23, preferably by spray ing from a suspension offluorescent material in liquid by means of an air brush. Alternately,however, the material may be,.applied by painting, or in any othersuitable manner.

In operation, an electron beam approaches my fluorescent structure alongthe arrow 22 and a portion of the electrons from said beam strike thefilm 23. In the absence of secondary emission from the film 23, theincident electrons cause;a negative charge to be accumulated upon thesurface of the insulating material 23.. certain other electronspenetrate the opening 25 between the wires 2| and strike the interiorsurface 25 of the window 6, upon which an additional negative charge isdeposited by the incident electrons. This charging procedure justdescribed wili in general occur only at the moment operating potentialsare applied to the tube after a substantial period of inactivity, andthe charges on the film 23 and the surface 26 will be built upsubstantially instantaneously. Following suchcharging, subsequentelectrons approaching along the arrow 22 will be given a component ofvelocity radial]; inward toward the axis of the opening 25 by thenegative charges existing on the surface of the film 23, so thatsubstantially all electrons in the beam will be caused to penetrate theopening 25 with only enough electrons striking the surface of the film23 to maintain the charge thereon at its maximum value. The remainingelectrons which penetrate the opening 25 will approach the window 25,but, because'of the charge existing upon that window, will be repelledin a direction corre: spending to the lines 21, following which pathsthe electrons will strike the fluorescent materiai 22 where their energywill be largely consumed and fluorescent light will be emitted; Thus,fluorescence is excited by an electron beam following the arrow 22, thefluorescent light being produced on a surface of fluorescent materialadapted to be viewed on the side upon which it was bombarded.

In order better to dissipate an electric charge which might otherwise beaccumulated on the surface of the fluorescent material 25, I have foundit desirable to use a fluorescent material having substantial ability toemit secondary electrons, and to leave a portion 2e oi. the surface ofthe wires 2| exposed so that secondary electrons emitted from thesurface 24 may beattracted along the lines 29 any, drawn to the wires 2|whence they may bei trried away through the wire mesh to appropriateconducting electrodes.

In order to ensure that all electrons which pass a given opening willstrike the fluorescent material on the wires adjacent that opening, itis important that the screen I be mounted in close proximity with thewindow 6. For example, I have found it advisable in many cases to mountthe screen I; with a spacing of less than one millimeter arid stillsmaller spacing is often desirabie. On 'the other hand, larger spacingis sometimes permissible because of the relatieely stiff space chargewhich builds up adjacent the inner surface of the window 6.

It may be seen that it lies' within the scope of my invention to providea separate member to perform the electron-repelling function of thewindow 6 when the characteristics of the glass of the window 5 are notwholly satisfactory for the required purpose. Forexample, an additionalplate of mica, or other material, may be carried on the ring 2 in aposition adjacent and parallel to the screen I and betweenv the screen Iand the window 6; and in this case, the addi tional plate performs theeiectrical function of the window 6.

In some instances, moreover, it is possible to provide a screen Iwithout the insulating material 23, in *which case it is advisable tomaintain the ratio of the diameter of the openings 25 to the diameter ofthe wires 2| at a value somewhat smaller than is desirable in thepresence of the insulating material. In this case a certain quantity ofthe bombarding electrons strike the exposed surface of the wires 2|, butthe preponderance of the electrons penetrate the openings 25 and areconcentrated with greater specific density upon the surface of thefluorescent material 24 because of the smaller area of the surface 24.In the absence of the insulating material film 23, it is not importantthat care be used in preventing the deposition of fluorescent 'materialon'the surface of the screen 'adjacentthe gun; actuaily the entiresurface of each of the wires 2| may be coated by dipping, painting,spraying or otherwise applying fluorescent material.

Still further, it is possible in some cases to dispense with theelectron repelling surface 26 adjacent the screen, in which case,advantage is taken of the relatively great space charge whichaccumulates in the space adjacent the screen on the side remote from theelectron gun. Thus it is clear that many modifications are possiblewithout departing from the spirit of my invention, and suchmodifications should not be construed as limiting the scope of theclaims which follow.

I claim:

1. In a cathode ray tube, a fluorescent screen and an electron gunadapted to project electrons toward said screen, said screen comprisinga reticulated metallic member, fluorescent material on the surface ofsaid member remote from said gun, insulating material on the surface ofsaid member adjacent said gun, the sur face within the reticulations ofsaid member being uncoatcd metal.

2. In a cathode ray tube, a fluorescent screen comprising a reticulatedmetallic member, fluorescent material on the surface of said screen, anelectron gun adapted to project electrons toward said screen, atransparent window adjacent said screen, said window adapted to bebombarded by a portion of the electrons penetrating the reticulations ofsaid screen, said window adapted to hold an electrical charge depositedby said incident electrons whereby the remainder of said penetratingelectrons may be repelled to the surface of said screen adjacent saidwindow.

3. In a cathode ray tube, a fluorescent screen comprising a reticulatedmetallic member, an electron gun adapted to project electrons towardsaid screen, fluorescent material on the surface of said reticulatedmember remote from said gun, insulating material on the surface of saidreticulated member adjacent said gun, said insulating material, adaptedto receive an electrical charge from a portion of the electrons fromsaid gun whereby the remainder of the electrons from said gun may berepelled from said insulating material and caused to penetrate thereticulations of said screen, a transparent window adjacent said screenon the side remote from said gun, said window adapted to be bombarded bya portion of the electrons penetrating the reticulations of said screen,said window adapted to hold an electrical charge deposited by saidincident electrons whereby the remainder of said penetrating electronsmay be repelled to the surface of said screen adjacent said window.

4. An electron discharge device comprising a tube provided with a screenand with means in front of said screen for developing a ray of electronsand directing the ray at the adjacent side of said screen, said screenbeing provided with relatively small openings through itself to permitfree passage of electrons of said ray in direct paths from said means tothe remote side of said screen by way of said openings, said screenbeing characterized by the fact that the individual elemental areas onthe rear side thereof become fluorescent upon bombardment by theelectrons of said ray which pass directly by way of said openings to therear side of said screen.

5. In apparatus for producing radiation by electronic bombardment, anenvelope, a screen mounted within said envelope having fluorescentmaterial upon one surface thereof and openings therethrough, and anelectron gun positioned upon the other side of said screen to projectelectrons toward said screen and through said openings, said envelopehaving means associated therewith to deflect electrons toward saidfluorescent material after they pass through said openings.

6. In apparatus for producing radiation by electronic bombardment, thecombination of a inminescent screen having openings therethrough, anelectron gun positioned on one side of said screen to project electronstoward said screen and through said openings, and controlling means forcausing said electrons to impinge the surface of said screen afterpassing through said openings.

7. In apparatus for producing radiation by electronic bombardment, thecombination of a luminescent screen having openings therethrough, anelectron gun positioned upon one side of said screen to projectelectrons toward said screen and through said openings, controllingmeans for causing said electrons to impinge the surface of said screenafter passing through said openings, and luminescent material on thesurface of said screen remote from said gun and so positioned that it isactivated by the electrons.

JOHN C. BATCHELOR.

